This invention relates to a kinescope vertical deflection circuit, and more particularly to a crossover distortion control arrangement. Vertical crossover distortion appears on a television kinescope or picture tube as a horizontal white bar across the center of the raster. Crossover distortion is caused by nonlinearity of the vertical deflection sawtooth waveform. Crossover distortion occurs most noticeably in vertical deflection arrangements with two active output devices which conduct alternately during the vertical scan interval. The crossover distortion arises in the region in which the second active output device is taking over the load from the first active output device, near zero deflection current. Deflection arrangements having two active output devices include conventional Class B amplifiers, Class D amplifiers and switched mode vertical deflection arrangements, an example of the latter being in accordance with copending U.S. application Ser. No. 595,809 filed July 14, 1975 for Peter E. Haferl and entitled "VERTICAL DEFLECTION SYSTEM".
The underlying cause for the deflection current distortion at a crossover differs between the three systems mentioned. In the Class B system, crossover distortion occurs because of the nonlinear transconductance (base-emitter voltage to collector-current transfer function) of the output transistors near the zero collector current operating point. Crossover distortion in a Class B system can generally be reduced to an acceptable level by providing a finite minimum (idling) current through the output transistors, in conjunction with degenerative amplitude-dependent feedback around the amplifier. In the Class D vertical defection system, in which constant amplitude square-wave voltage pulses of varying duty cycle are applied to the vertical deflection winding, crossover distortion results from the finite voltage drop of the diodes included as part of the output switches. Crossover distortion in Class D systems can be minimized by appropriate impedance transformations or by reverse conduction of the output switch transistors as described in U.S. Pat. No. 3,939,380 in the name of John Charles Peer and entitled "CLASS D AMPLIFIER".
In a switched mode vertical deflection system as described in the aforementioned copending application, crossover distortion results from the shape of the retrace pulses of the television horizontal deflection system. The switched mode vertical deflection system derives its power directly from the horizontal retract pulses. The retrace pulses are gated and current pulses derived therefrom and having amplitude and polarity varying at the vertical rate are used to charge a capacitor. A vertical deflection winding is connected across the capacitor and the capacitor discharge current through the deflection winding is the sawtooth vertical deflection current.
Control of the two opposite-polarity current pulses from which the sawtooth deflection current is derived in the switched mode vertical deflection system is by means of a thyristor switch for each current polarity. During the first portion of the vertical deflection scan interval, a first thyristor switch is gated on at a time increasingly delayed relative to the leading edges of the horizontal retrace pulses. This results in the application during the first half of the vertical scan interval of current pulses of diminishing magnitude in a first polarity to the sawtooth capacitor. During the second half of the vertical scan interval, the first thyristor is not gated, but the second thyristor is gated on at a time during the retrace interval which is progressively advanced during the remainder of the scan interval. This results in application of current pulses of increasing magnitude to the sawtooth capacitor and consequent sawtooth deflection current.
Since the vertical deflection current in the switched mode vertical deflection circuit is derived by time-dependent gating directly from the horizontal retrace pulse, time-dependent variations in the amplitude of the horizontal retrace pulse will result in nonlinearity of the derived current. In particular, near the center of the vertical scan, when the first switch is gated on near the end of the horizontal retrace interval so as to provide a current pulse of short duration representative of low average deflection current, the small magnitude of the sinusoidal horizontal retrace voltage pulse near the end of the retrace pulse interval will result in a gated current pulse of disproportionately small magnitude. Thus, the vertical deflection current near the center of the vertical scan interval may reach zero current too early, resulting in crossover distortion apparent as a bright white line on the raster.
Crossover distortion in a switched mode vertical deflection circuit cannot be compensated by amplitude-dependent degenerative feedback because the trailing edge of the horizontal retrace pulse contains insufficient energy to supply the required average power. To overcome this problem, the time relative to the horizontal retrace interval at which the first and second switches are gted on near the center of the vertical scan interval in the prior art is moved to a point at which the horizontal retrace pulse contains appreciable energy. This results in an "overlapping" mode of operation about the center of trace, in which the second switch is gated into conduction during horizontal retrace intervals before the center of the vertical scan interval and the first switch ceases conduction at a time after the center of the vertical scan interval. This overlapping operation increases the power capability of the switched mode vertical deflection circuit near the center of the vertical scan. The overlapping operation of the switches serves to compensate for the non-ideal waveshape of the horizontal retrace voltage pulse so as to produce linear current operation through the center region of the vertical scan.
Overlap of the conduction periods of the first and second switches in excess of the amount required to make the crossover distortion insignificant results in excessive power dissipation or may lead to cessation of operation of the horizontal deflection circuit, because the overlapping conduction causes a large circulating current through the vertical deflection circuit and presents an equivalent short-circuited load to the horizontal deflection circuit. Adjustment of the overlap of the conduction times of the vertical control switches for optimum contemporaneity in the aforementioned copending application is manually preset for linear deflection without excessive dissipation.